Hurricane Irma's eyewall reaches Florida Keys

The eye of the Category 4 storm was 24km southeast of Key West, bringing winds up to 200km/h and threatening dangerous storm surges.

"This is an extremely dangerous and life-threatening situation!" the National Weather Service in Key West had said, urging those who had not heeded dire warnings to evacuate and take shelter "now to protect your life!"

The hurricane is threatening almost the entire southeastern US state of Florida after cutting a deadly path of destruction through the Caribbean.

Tens of thousands of Floridians were hunkering down in shelters for a direct hit from the monster storm, after more than 6.3 million - nearly a third of the state's population - were ordered to evacuate.

For those still at home, it was already too late to escape the wrath of what could be the worst hurricane in storm-prone Florida.

Florida Power and Light Company said that about 430,000 customers were without power on Sunday morning.

Miami-Dade County had the most outages with about 250,000. Broward County had 130,000 outages. Palm Beach County had more than 40,000 outages.

The utility company said that it has mobilised crews and is working to restore power as it can.

Mandatory evacuation

MacDill Air Force Base, the military installation home to US Central Command, issued mandatory evacuation orders with the eye of the storm expected to pass over its home city of Tampa early on Monday.

The Kennedy Space Center was also closed.

After blasting through the nearby Cuban coastline, Irma weakened from a maximum-strength Category 5 to a Category 3 storm, but then strengthened again to a Category 4, with 210km/h winds, as it approached south Florida.

There was a serious threat of flooding from storm surges of up to 4.5 metres along Florida's west coast - enough to cover a house.

READ MORE: Irma lashes Florida, leaving thousands without power

The hurricane ripped roofs off houses, collapsed buildings and flooded hundreds of kilometres of coastline as it raked Cuba's northern coast after devastating islands the length of the Caribbean in a trail of destruction that has left 25 people dead so far.

Terrified Cubans who rode out Irma in coastal towns after the storm made landfall on Friday on the Camaguey archipelago reported "deafening" winds, uprooted trees and power lines, and blown rooftops.

"We feel that we have done all that we can ... whether this storm hits us as a Category 5, 4, or what they're saying since it's moved away - a Category 1 or 2. It doesn't matter. We made the right call."

Mass exodus

In Florida, cities on both the east and west coasts took on the appearance of ghost towns, as nervous residents heeded insistent evacuation orders.

The storm was expected to move along or near Florida's southwest Gulf coast by Sunday afternoon.

But Irma is so wide that authorities were bracing for destructive storm surges on both coasts and the Keys, the chain of low-lying islands that stretch south of Miami toward Cuba.